sessionmanagement: handle suspendThenHibernate
Needed for powerdevil#28.
!3222 (merged) already took care of canSuspendThenHibernate
; this implements the actual suspend-then-hibernating.
Tested by invoking through KRunner with https://invent.kde.org/plasma/plasma-workspace/-/commits/work/natalie/powerdevilrunner-suspend-then-hibernate; to be MRed separately once this one is through.
Edited by Natalie Clarius